TET-1 (Technologie Erprobungs Träger-1) is a German technology demonstration microsatellite of DLR (German Aerospace Center) within its OOV (On-Orbit Verification) program. Project funding is provided by the German Ministry for Economics and Technology (Bundesministerium für Wirtschaft und Technologie). The overall objective is to provide industry and research institutes with adequate means for the in-flight validation of space technology. Certain programmatic rules were established for the space segment and the ground segment to realize TET-1 as a low-cost mission within a relatively short timeframe under the leadership of an industrial space company as prime contractor. 1)2)3)4)5)6)7)

Flight opportunities for technology demonstration and verification should ideally be provided on a regular basis in a cost efficient and safe manner. A market survey of German industries' and institutes' technologies has shown that about 75% of the experiments can be verified using a microsatellite.

The OOV-Program is thus structured into two main parts with respect to the flight opportunities offered. The first comprises the microsatellites TET with a planned flight opportunity every two years. For payloads which do not fit on TET microsatellite concept, DLR will cooperate with national and international partners to provide flight opportunities on other carriers. 8)9)

The small satellites in this series are largely based on the flight-proven BIRD (Bi-Spectral Infra-Red Detection) spacecraft bus launched in 2001. At a size of about 65 cm x 55 cm x 88 cm and a total mass of ~ 120 kg, the TET spacecraft can handle a payload of up to 50 kg in an envelope of 460 mm x 460 mm x 428 mm (Figure 2).

In July 2008, DLR awarded the prime contract to Kayser-Threde GmbH of Munich (a company of OHB-System, Bremen) for the space and ground segment as well as for the launch services of TET-1. The microsatellite bus was designed and developed at AstroFein (Astro- und Feinwerktechnik Adlershof GmbH), Berlin in a subcontract to Kayser-Threde.

The modular multimission spacecraft bus consists of three segments: service, electronics, and payload. In the Service Segment a battery, reaction wheels, the PCU (Power Control Unit) and laser gyro are installed. The Electronics Segment contains the SBC (Satellite Bus Computer) and the payload supply system. In the Payload Segment are the experiments and also satellite bus components (star sensors, magnetic field sensors and antennas (low-gain and GPS). 10)11)12)13)14)15)16)

Figure 2: The envelope of the generic TET spacecraft bus with the empty payload segment on top (image credit: AstroFein)

A special point in the design of the satellite bus was the interface between satellite bus and payload. To support different kinds of missions, the system contains the nominal satellite bus and a PSS (Payload Supply System). This payload supply system is on its payload interface side adaptable to the data (SpaceWire, RS422/485, CAN-Bus, etc.) and power interface requirements, data storage requirements and payload control requirements.

The nominal satellite bus will remain unchanged for different missions, but of course can be adapted in parts, like an upgrade to X-band system if higher data rates are required. The PCBs (Printed Circuit Boards) of the PSS will be adapted for every new payload accommodation.

The Service Segment contains the PCU, two IMUs, four reaction wheels and two battery stacks. The Electronics Segment comprises the PSS (Payload Support System), the SBC (Spacecraft Bus Computer), the PDU (Power Distribution Unit), data processing boards of the redundant sun sensor system, the redundant GPS and the driver electronics of the redundant magnetic coil system. All of these components of the Electronics Segment are designed as PCBs in Europe Card size (160 mm x 100 mm, max. 15 PCBs for PSS and 15 PCBs for the satellite bus components) and are connected via two backplanes. Additional to that the complete TM/TC hardware is integrated into the Electronics Segment, except the two low gain antennas (for omni-directional communication).

The PSS (Payload Support System) provides the control function and physical interface between the spacecraft bus and the experiments. The PSS consists of three elements which are realized on PCBs (Printed Circuit Board) connected to the backplane:

1) The power supplies responsible for the supply of the experiments and the PSS itself

2) The processor boards that are used to control all PSS activities

3) The I/O boards that provide the interfaces to the experiments.

The functions provided by the software executed on the PSS processor cover the following tasks:

- Control of experiments with minimal built-in processing logic

- Acquisition of housekeeping data of the experiments and the PSS itself

- Acquisition of measurement data of the experiments

- Temporary storage of all acquired measurement and housekeeping data

- Formatting and packaging of data according CCSDS for downlink

- Reception and execution of commands from the spacecraft bus controller

- Control of PSS devices as switches, circuit breakers, etc.

- Management of thermal control of experiments and PSS

- Boot, self-test and software update management

- Failure detection, isolation and recovery, redundancy management.

AOCS (Attitude and Orbit Control Subsystem): The AOCS uses a redundant set of star sensors, gyroscopes, magnetometers, and sun sensors for attitude sensing. Actuation is provided by 4 reaction wheels and a magnetic coil system. The main design driving requirements for the fault-tolerant AOCS are: 17)18)19)20)

The main torque actuators of TET-1 are four precise RWs (Reaction Wheels) in hot redundant tetrahedron configuration. This redundant configuration allows the RWs to work in lower rpm level which leads to less mechanical abrasion. The three internally redundant MCSs (Magnetic Coil Systems) are drivable independently from each other. 21)22)

Figure 5: Overview of the AOCS elements of TET-1 (image credit: DLR)

All sensors for attitude and orbit determination are available as redundant systems as well. The ASC (Autonomous Stellar Compass) of DTU is the primary sensor for precise attitude determination (< 24 arcsec) and comprises a cold redundant DPU and two switchable CHUs. The CHUs boresight axes are tilted 70º away from each other to avoid simultaneous sun, moon or earth blinding. - The CSS (Coarse Sun Sensor) system consists of four internally redundant analog sensor heads which are mounted at the outermost area of the satellite's outer surface to ensure maximum signal availability. The analog signal processing board is arranged redundantly as well.

A further reference sensor is the fluxgate MFS (cold redundant) which is needed for magnetic field determination and MCS control input. The angular rate is determined besides the ASC mainly by the cold redundant IMU. - The orbit determination is performed by a cold redundant GPS system. The position and time data obtained by the GPS is introduced into the orbit estimator in the ONS (Onboard Navigation System) module.

The robustness of the AOCS is realized by redundancy of H/W components and a "configuration management system" to handle the redundancy. The autonomy requires onboard failure detection and diagnosis. Another design aspect for robustness is a robust control loop to react against perturbation and component anomaly with sufficient stability margin. Also essential for a seamless filtering and processing of AOCS data is the autonomous monitoring of the "timings" of all AOCS threads and processes. 23)24)

Figure 6: Overview of the S/W and H/W components of the AOCS (image credit: DLR)

The AOCS S/W is build up as an object oriented and modular application with a static architecture. The break down into several layers (Figure 7) allows an autonomous and robust handling of each feature and was extensively proven during the BIRD mission. The S/W runs as an application for the real time operating system BOSS (timing accuracy 1 ms) on the SBC.

- The lowest layer consists of the interface modules to the hardware components and the ONS module. The low level data processing and formatting of actuator and sensor data takes place in this layer.

- A layer above resides the so called EPC (Estimator, Predictor, Controller) layer, in which the actual control task takes place. The encapsulating EPC module is a combination of state estimation, prediction (Kalman filter) and state control. The estimator, predictor and controller modules form together with the mode processor the core of the control S/W.

- The topmost layer contains the TMTC (telemetry and telecommand) interface of the AOCS and its components. A very elementary role in the FDIR is taken by the surveillance module, which is responsible for monitoring the thread timing.

The redundant GPS system is referred to as AGPS-1 (Astrofein GPS-1) which is being space-qualified on this mission. AGPS-1 is based on the the Phoenix-Sensor and calculation algorithm of DLR/GSOC and fulfills the following technical specifications: 25)

The system offers a built-in orbit propagator to aid the initial acquisition and to allow a short time-to-first-fix. Due to a separate provided memory for the GPS catalog, the system can make a warm start, also in the case when it was nearly shut down.

EPS (Electric Power Subsystem): The EPS consists of a triple junction GaAs solar array, PCDU (Power Control and Distribution Units), and a NiH2 cell battery of 240 Ah capacity. The PCDU in turn consists of PCU+PDU. The solar array has one fixed panel and two deployable panels. Electrical power of 220 W (maximum power point) is provided. Use of regulated and unregulated power modules.

Figure 8: Photo of the NiH2 cells and PCDU (image credit: AstroFein)

The SBC (Spacecraft Bus Computer) controls all activities of the subsystems and the satellite bus. The SBC consists of 4 identical boards (2 in hot, 2 in cold redundancy) and watchdog circuits for failure detection and recovery. The architecture of the redundant SBC boards (nodes) is totally symmetric; each board is able to execute all control tasks. One node (the worker) is controlling the satellite while a second node (supervisor) is supervising the correct operation of the worker node. The two other node computers are spare components and are disconnected.

TCS (Thermal Control Subsystem): A semi-active TCS was developed to keep the temperature of the satellite and all of its components within the parameters of normal operating temperatures. It consists mainly of a MLI (Multi-Layer Insulation), heat pipes, the radiator, temperature sensors and heaters. The MLI detaches the satellite thermally from its environment, which means that an exchange can only take place via the radiator. The radiator is coated with a special white paint, which remains stable over long periods of time and has a good absorption-emission ratio. The radiator is mounted on the underside of the satellite, which generally does not face either the sun or Earth.- The heat pipes lead waste heat emanating from the payload directly to the radiator to avoid compromising the satellite bus components. Additional heat can be generated by the heater, should the satellite cool down excessively.

RF communications: An S-band system is used with hot redundant receivers and cold redundant transmitters. The downlink transmission rates are 137.5 kbit/s or 2.2 Mbit/s. The transmitter and receiver channels are redundant and can be switched to the omni-directional low gain antenna system or to the high gain antenna.

The system can emit the telemetry via the redundant transfer switches and the omni-directional low gain antennas, or the directional high gain antenna. The entire system is designed in accordance with the international CCSDS standard (Consultative Committee for Space Data Systems), and in its current configuration allows uplink speeds of 4 kbit/s and downlink speeds of 2.2 Mbit/s.

In 2010, the TET bus is a commercially available product of Astro- und Feinwerktechnik that is offered to the microsatellite community. Due to market requests there are plans for a bus upgrade to include the following items (Ref.10) :

- Higher payload mass (≥ 70 kg) and increased envelope

- More power for the payload

- X-band downlink for payload data

- Propulsion subsystem (as optional piggyback system, will be part of payload mass).

Figure 10: Photo of TET-1 in January 2011 prior to shipment to the launch site (image credit: Kayser-Threde)

Launch: The TET-1 spacecraft was launched on July 22, 2012 as a secondary payload to the Kanopus-V-N 1 primary spacecraft of Roskosmos/Roshydromet/Planeta on a Soyuz-FG Fregat launch vehicle. The launch site was the Baikonur Cosmodrome, Kazakhstan. The launch provider was Starsem. 26)27)

Note: The TET-1 spacecraft has been ready for launch since the beginning of 2011. The last launch date for Kanopus-Vulkan was scheduled for September 2011. However, the Soyuz launch vehicle experienced a failure on August 24, 2011, carrying a Progress M-12M capsule filled with supplies for the ISS (International Space Station). This event resulted in a failure investigation and in a follow-up delay of all planned Soyuz launches. - In particular, the long launch delay was due to the still incomplete readiness status of the primary spacecraft, a fate often experienced by the secondary payloads.

Kanopus-V-N 1, BelKa-2 and TET-1 were released into an orbit of ~ 510 km. Afterwards, Fregat had to maneuver to a higher ~800 km orbit to deploy Zond-PP and the exactView-1 payloads. All satellites were successfully deployed and Fregat made a deorbit maneuver.

• December 20, 2017: Emergency services in the US state of California are still fighting fierce forest fires. Severe drought and strong winds have allowed the fires to spread. The FireBIRD (Fire Bispectral InfraRed Detector) mission run by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) consists of a pair of satellites – TET-1 (Technology Experiment Carrier) and BIROS (Bispectral Infrared Optical System). These detect high-temperature events from space. On Sunday 10 December TET-1 registered a major seat of fire near the town of Ventura, north of Los Angeles, on the US Pacific coast. Further data was recorded over the days that followed. "FireBIRD's precise systems allow us to detect incipient changes to fires with accuracy," says Winfried Halle, FireBIRD project manager at the DLR Institute of Optical Sensor Systems, adding, "This can help us predict the behavior and development of major fires more effectively in future." 28)

Figure 12: The TET-1 satellite has been recording the seat of a fire on the US Pacific coast for several days. The precise sensor systems of FireBIRD's satellite mission allow the spread and radiation energy of the fire to be measured with accuracy (image credit: DLR)

- Energetic fires: After analyzing initial imagery, DLR researchers realized that in central areas the fire burned more intensely and thus radiated more energy than was the case with the major forest fires that occurred in Chile, Portugal and British Columbia in 2016 and 2017. Scientists calculated a radiation energy of up to 18 megawatts for individual zones within the overall area affected by fires in California. The radiation energy of the aforementioned fires elsewhere in the world was only around half as high.

- The data showed that over the course of the week the individual fire zones varied greatly in intensity and spatial extent. The largest contiguous area of fire was recorded on 10 December, while the largest number of individual, intensely burning fires was registered on 12 December. Over subsequent days the areas of intense fire declined. However, smaller fires increasingly flared up in other places. The recordings showed that, put together, the fires devastated a large area, hampering fire-fighting operations. "In Europe there is no other comparable satellite mission that is capable of measuring the radiation energy of fires with such precision," notes Halle.

- For large fire events such as the forest fires that raged in Chile in January 2017, the DLR Center for Satellite-Based Crisis Information (Zentrum für Satellitengestützte Kriseninformation; ZKI) provides data from FireBIRD to local authorities. In crisis situations the ZKI puts the relevant satellite data together in such a way that it can be optimally used by situation centers, public authorities, relief organizations and policy makers. The ZKI created an overview map of forest fires in California using FireBIRD data.

- About the FireBIRD mission: The DLR FireBIRD mission consists of two satellites, TET-1 and BIROS (Bispectral Infrared Optical System). They are both based on the small satellite BIRD (operational from 2001 to 2004), which was developed by the DLR Institute of Optical Sensor Systems. A similar system of infrared cameras is fitted to both of these satellites. TET-1 has been orbiting Earth since 2012, searching for fires and other high-temperature events. TET-1's 'brother', the small satellite BIROS, built in Berlin Adlershof, has also been in orbit since 2016, adopting an open constellation to support TET-1 on its important mission.

- The satellite data is mainly received at the DLR ground station in Neustrelitz and then processed, archived and made available worldwide for scientific purposes by DFD (German Remote Sensing Data Center). The FireBIRD satellites are operated and controlled by GSOC (German Space Operations Center) within DLR's Space Operations in Oberpfaffenhofen.

• April 2017: SST (Sea Surface Temperature) is an important parameter for meteorology, climatology and oceanography and is used for models and forecasts. It represents the interface between ocean and atmosphere and describes the coupling of these two systems. Furthermore, the increase of the SST is seen as an indication of the global climate change and the warming of the oceans. — For these reasons a continuous and global observation of SST by satellite imagery is necessary. Besides the advantages of measurements by satellites, there is one essential disadvantage in comparison to the in-situ measurements: The desired quantities can only be determined indirectly. The parameters like the SST have to be calculated from radiation measured by the satellite sensor. This procedure is called inverting. The accurate determination of atmospheric influences is another difficulty. A signal from the ground must first cross the Earth's atmosphere, before it can be detected by the satellite. The determination of the SST is mostly influenced by water vapor in the atmosphere. 29)

- The TET-1 midwave (3.4 –4.2 µm) and longwave (8.5 –9.3 µm) infrared data in was used to estimate the SST. The TET-1 satellite sensor was developed by DLR in the project group FireBIRD and was launched in 2012. Meanwhile it reached its operational phase and is mainly used for forest fire monitoring and high temperature events detection. Further applications, e.g. SST, could follow in the future. The beneficial feature of TET-1 for this purpose is its ground sampling distance of 160 m. By using this high spatial resolution it is possible to monitor small-scale basins like lakes and coastal waters.

- Methodology: The determination of the SST by optimal estimation requires specific prior knowledge. Firstly, a forward model, called look-up table, which consists of radiative transfer simulations by MODTRAN, provides information for the calculations. Apart from that, the GFS (Global Forecast System) of the NCEP (National Center for Environmental Prediction) is used.

- LUT (Look-Up Table) settings: The LUT consists of individual radiative transfer simulations for the two infrared channels. It is possible to calculate the expected at-satellite-radiations depending on the different atmospheric states and situation on the ground. For simulating the atmospheric states, some model parameters were modified. These are: SST temperature at ground level, water vapor content in the atmosphere, ozone content in the atmosphere and optical range (associated with the aerosol content). In order to analyze the influence of the technical parameters such as the viewing angle, these were also varied. Constant values were given for the emissivity (ε = 0.95), carbon dioxide content (CO2 =400 ppm) and the composition of the atmospheric aerosols. Volcanic events were not taken into consideration. Overall the LUT is made of 70,000 radiative transfer simulations and serves as forward model of the optimal estimation processor.

Figure 13 on the left-hand side shows an example for an input dataset of channel MWIR and on the right-hand side the output of SST from the optimal estimation. The SST ranges between 282 K and 289 K and includes a huge north-south gradient. Cooler structures can be identified as islands and clouds. Small-scale turbulences can be resolved in the input datasets as well as in the estimated SST.

Figure 14 (left side) shows the estimated uncertainty [K]. It composes of the measurement accuracy and prior knowledge and varies between 1.75 K and 1.92 K. The uncertainty increases by higher water vapor content in the atmosphere, e.g. in the northern part of the scene. In the areas covered by clouds the processor cannot find a solution and rejects the convergence criterion (not shown).

The verification of the results is done with SST data from GHRSST (Group for High Resolution Sea Surface Temperature). The pixel-wise difference between these two SST products is presented in Figure 14 (right side). Mainly we can observe a positive difference, which increases with the higher water vapor content. Only clouded regions and land areas have negative differences, but there the processor cannot find a permissible solution. As further investigated scenarios show a similar behavior, a systematic deviation could be assumed.

In summary, the results suggest that with a further optimization of the estimation, application of the TET-1 data in this field could provide satisfactory results. The estimation using two infrared channels generates a plausible accuracy about 1.7 K to 1.9 K. Clouds and the high water vapor concentration proved to be problematic for the processor. Verification of the calculation results by another SST product (GHRSST) reveals a systematic deviation that should be further analyzed. The high ground sampling resolution allows observing also small-scale waters like lakes and coastal waters.

In the future, the study and upcoming applications could be extended by using the data from another satellite sensor of the FireBIRD mission, BIROS, launched in June 2016. It was equipped with a camera system with the same parameters and operates in the same time shifted orbit. These characteristics enable making comparable measurements with the two satellites for the future development and application of the described algorithm.

• January 31, 2017: Expansive forest fires have raged through Chile for some weeks due to a long dry spell. On 25 January 2017, the Chilean National Office for Emergency (Oficina Nacional de Emergencia del Ministerio del Interior; ONEMI) activated the International Charter "Space and Major Disasters" to obtain up-to-date situation images of the disaster area to assist emergency services. The Center for Satellite Based Crisis Information (Zentrum für satellitengestützte Kriseninformation; ZKI) of DLR/DFD (German Aerospace Center/Deutsches Fernerkundungsdatenzentrum) was tasked with coordinating the entire Charter activation. As part of their Charter activities, the staff at ZKI requested, among other things, that the Institute of Optical Sensor Systems provide data from the DLR satellite TET-1, which is part of the FireBIRD mission. The smooth internal coordination within DLR ensured that only a few hours passed from the order to the planning, sending of satellite commands, and data download and processing, before ZKI was able to send the first situation map of the fire hotspots to Chile. 30)31)

- Raging fires with immense radiative power: The fires in Chile are the most devastating in years. Since the outbreak of the first fires, emergency services have counted 135 active fires, 55 of which are under control and 11 have been extinguished. So far, the fires have affected a surface area of over 366,000 hectares (3660 km2) and destroyed 1047 buildings. The satellite image taken with the mid-infrared channel of the TET-1 camera shows an area 35 km wide and 60 km long. The yellow to red areas indicate the fire zones; the different colors indicate the respective fire radiative power. Yellow areas are fire sources with low radiative power, for instance in shrubs. In contrast, the red areas denote particularly highly radiative power typical of entire trees that have caught fire. In addition to the huge fire zones close to the cities of Constitución and Concepción, numerous smaller fires are visible throughout the entire country.

- The data provided by the small satellite TET-1 was automatically processed to create so-called fire products, in which the radiative power of fire was inferred in addition to thermal anomalies (conspicuous deviations in the land surface temperatures). The measurements were reviewed by sensor specialists at the DLR Institute of Optical Sensor Systems, who identified extreme values of up to 24 GW (gigawatt) in their analyses of fire sites. The staff at ZKI were able to use this information to prepare the up-to-date situation maps (Figure 15).

- International disaster relief: A heatwave conflating with strong winds led to several multiplications of fires in a short period of time, prompting the Chilean authorities to declare a disaster situation on 20 January 2017. The prevailing drought and the weather forecast for the coming days, with temperatures topping 39 ºC and accompanied by strong winds, are expected to exacerbate the situation even more. Four thousand people have already been evacuated from the affected areas. All available resources nationwide have been deployed. In addition, Mexico, France, Russia and the United States have provided extra manpower. France and Russia are also contributing to situation analysis within the framework of the ZKI-coordinated Charter activities.

- ZKI (Center for Satellite Based Crisis Information) at DLR supports emergency services by providing rapid satellite image maps for use in natural disasters and humanitarian emergencies. Satellite data is recorded, analyzed and compiled to produce needs-based, topical maps and visualizations on very short notice, especially within the framework of the International Charter "Space and Major Disasters". ZKI is integrated in national and international networks, and its staff contribute to the UN-SPIDER (United Nations Platform for Space-based Information for Disaster Management and Emergency Response).

- The satellite data are mainly received at the DLR ground station in Neustrelitz and then processed, archived and made available worldwide for scientific purposes by the German Remote Sensing Data Center (DFD). The FireBIRD satellites are operated and controlled by the GSOC (German Space Operations Center) within DLR's Space Operations in Oberpfaffenhofen.

• October 30, 2015: Indonesia is on fire – the island state is currently facing a bitter struggle against forest and peat fires on Sumatra and Borneo, most likely caused by illegal 'slash and burn' farming to clear the land for palm oil or timber plantations. The extremely dry conditions resulting from the El Niño weather phenomenon exacerbate this problem. Reliable information on the source and spread of the fires is needed to initiate suitable countermeasures and to estimate the environmental consequences. Although satellite images can provide this information, the thick clouds of smoke concealing the exact locations of the fires often prevent them from delivering sharp pictures. This is why the microsatellite TET-1 (Technology Testbed-1), built by DLR, has been deployed. "The high sensitivity and spatial resolution of the camera installed on TET-1 allows us to detect and monitor very small fires, even through thick smoke," says Eckehard Lorenz from the DLR Institute of Optical Sensor Systems and Principal Investigator of the mission, who was involved in the development of the camera. 32)

Figure 17: Top: A series of TET-1 images of fires in Indonesia, acquired in September and October 2015; Bottom: equivalent MODIS Hotspot events (image credit: DLR)

- The fires in Indonesia are emitting large quantities of carbon that shroud the neighboring countries of Malaysia, Singapore and Thailand in smoke and obstruct fire detection from space. The Pollutant Standards Index indicates the quantity of atmospheric contaminants, and it clearly shows how dangerous the particulate pollution is for humans and the environment – the Province of Kalimantan logged a value of 1800 at the start of October, with 300 considered the threshold for seriously harmful air pollution.

- Although satellites operated by NASA, such as Aqua and Terra, can use their MODIS (Moderate Resolution Imaging Spectroradiometer) instruments to detect irregularities in fire and heat in their satellite images, the irregularities themselves must be sufficiently strong for the sensors to detect them. In some cases, clouds and smoke will influence or prevent the identification of fires. This is where DLR's TET-1 satellite can provide a solution: "What makes TET unique is the special design of its instruments. The two infrared cameras are able to detect and analyze objects on Earth using infrared radiation," explains Lorenz.

- Although the microsatellite cannot cover the same area as a larger satellite, its flexible control compensates for this to a certain degree. TET-1 delivers comprehensive data that scientists like Florian Siegert, a biologist at the GeoBio-Center at Ludwig Maximilian University in Munich, use in their work. Siegert analyses the impact of peat fires on the global climate and advises organizations such as Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) and the World Wildlife Fund (WWF) about the ecological state of Indonesia. His analyses of time series images help to identify the source of fires, their expansion patterns as they spread, as well as the speed at which the hotspots move. Researchers analyze the temperatures of the fires to detect their potential spread and to calculate their thermal output. They can therefore determine whether they are looking at a low-energy peat fire or a 'hot' fire spreading across vegetation. "The TET data and other field measurements conducted on the ground allow us to estimate the greenhouse gas emissions associated with peat and forest fires with much greater precision than before. This is an important contribution to researching the causes of climate change," explains Siegert. Therefore, a combination of two or more TET satellites could deliver important information and provide an early warning system for fires breaking out in the tropical belt.

- TET-1 has now become a part of the FireBIRD mission, which in the future will use two satellites working in tandem, orbiting Earth to acquire remote sensing data for the detection of fires. Their task is to identify and measure high temperature incidents and to deliver remote sensing data for scientific research conducted by DLR and other partners. DLR will launch the BIROS (Berlin Infrared Optical System) satellite in April 2016 to achieve these goals and will work with TET-1 to provide the necessary data. In particular, BIROS will be tasked with transmitting information on currently active or nascent fire sources directly to mobile communications devices. The satellite is equipped with a propulsion system that can be used to alter its constellation position with TET -1 to enable repeated imaging of a specific location from various angles (Ref. 32).

• May 2015: The TET-1 mission is operating 'nominally' in 2015 , completing its 3rd year on orbit in July. - More than 10 years after the launch of DLR's first small satellite BIRD, a follow-on project, called FireBird was started. Based on the success of the BIRD mission, the main scientific goal- the investigation of high temperature events and their impact on the climatic processes- will be continued, but in consideration to the advantages given by the operation of a constellation of two small satellites. The first of these satellites — TET-1, was launched on June 22, 2012. The launch of the second satellite, BIROS (Bi-spectral Infrared Optical System), is scheduled for spring 2016. 33)

• Since November 2014, the OOV-TET1 Satellite is operated by DLR as part of the FireBird mission. The detection of high temperature events is one of the main features of the TET-1 satellite. Therefore, the focus lies on the detection of fires and events with temperatures around 1000 K. In order to avoid saturation for events with high energy, two samples per ground pixel are taken, with the second sampling is performed using a shorter integration time. With TET-1, it is even possible to detect fires with areas much smaller than the actual ground mapped pixel size.34)

To achieve this sub-pixel accuracy, it is necessary to have at least two distinct infrared bands which are provided by TET. One band is sensitive in the mid infrared range (3.4 to 4.2 µm) where typical fires have their radiative maximum. The second band required is sensitive in the thermal infrared range (8.5 to 9.3 µm), providing the background temperature. Using these two infrared bands as input data for the dual-band approach, the effective temperature and the effective sub-pixel area of a detected high temperature event can be estimated simultaneously. Additionally, the red band of TET is used for basic classification for background pixel assessment and false alarm rejection e.g. due to sun light reflections.

Before applying the dual-band approach, the three different spectral bands have to be co-registered and potential fire pixels have to be identified. The red band and the thermal infrared band are mapped to the mid infrared band by a half-automatic method. A rough manual mapping is refined by a cross-correlation approach. After this, the three bands are co-registered with a precision of less than a pixel. Then, background pixels are determined by interpreting these three bands, e.g. classify and exclude clouds, sunglints, and hot objects. These background pixels define thresholds which are used to identify fire pixels. After clustering neighbor fire pixels, the dual-band method estimates fire temperature and area for all found fire clusters. Furthermore and finally, the fire radiative power (FRP) and geographic coordinates are calculated for every fire cluster. Since the active fire front covers often only a small fraction of a pixel less than 10% it is essential to have a good estimation of the background temperature. Canopy layers, e. g. forests and bush lands are often very heterogeneous, which can lead to larger margins in fire parameter estimations especially in the case of small fires.

- Validation activities: In order to obtain a correct interpretation of the radiances, it is essential to compare the fire parameters derived from the observations with parameters obtained from ground based measurements under defined conditions. For this, a test fire design has been made and deployed at the calibration side DEMMIN® (Durable Environmental Multidisciplinary Monitoring Information Network) in north-east Germany. A test burn has been performed on August 17, 2013. The aim of the test was to create a fire at the low detection threshold of the sensor with a well specified dimension and a homogeneous surface temperature. The estimated FRP, an abstract parameter to describe the fire with respect to biomass consumed, was 2.24 MW compared to 1.36 MW calculated.

In the time that the IR camera system aboard TET-1 has been active, approximately 1450 scenes have been captured containing wildfires and other HTE, which are volcanos and industrial sites, like power plants, offshore gas and oil rigs, refineries, and mine sites. HTE occur in all continents within a wide range of land cover types, from grasslands in South Africa, Eucalyptus forests in Australia, Boreal forests in Canada and even volcanos in Iceland.

- Detection of Hotspots in Australia: The climate in Australia is conducive to the spread of fire across the large regions of the continent. From the hot dry summers (November to March) in the southern temperate zones to the tropical dry season (April to September) in the north, fire events occur across the continent over the entire year. This provides an opportunity for the TET-1 satellite system and the FIREBIRD project in the future, with a target area with the potential to provide a wide range of fire types and sizes, over a wide range of surface land cover varieties.

Figure 19 shows a site west of Sydney and was the site of a catastrophic wildfire commencing between 17th and 20th October and ending on 19th November. This fire resulted in two people dying and 248 buildings destroyed, with a total cost of $94 million. In total, all of the fire instances in this area during this time burnt over 65,000 hectares.

- Monitoring Sea Surfaces Temperatures: Although monitoring of water surfaces is not the designed task of TET-1, a good example of different water brightness temperatures obtained from the LWIR band is shown in Figure 20. The dark blue areas indicate upwelling cold water in the Baltic Sea between Germany and Denmark. In order to separate flat water areas from land additional information is needed, such as a water mask obtained from the NIR band. Brightness temperatures might be also used to detect maritime pollution, e. g. oil spills, since they affect the emissivity of the surface in the thermal infrared range. This subject, however, hasn't been investigated due to a lack of either clear scenes and, fortunately lack of polluted areas to observe.

The TET-1 satellite is powered by nickel-hydrogen (NiH2) batteries during eclipse phases of the orbit. Although unexpectedly confronted with high battery temperatures after launch, the OOV (On-Orbit Verification) mission could successfully be completed.

The operations team became aware of several issues shortly before and after the launch which created some challenges for battery operation. Some battery cells had undergone reversal prior to launch which seems to have created an imbalance between them. Also, the overall temperature within the satellite and that of the batteries turned out to be higher than predicted and the battery voltage slightly exceeded the limit for payload operations.

Regulating the battery temperature was at first attempted by lowering the amount of charge put into the battery. However, this could not be done indefinitely due to worries of battery damage (memory effect) and the constricted charge available for payload operations. Hence, other approaches were employed. The SPRM (Sun Pointing Rotate Mode was one of the methods used. The satellite was reoriented in such a way that the bus radiator was pointed away from the Earth and the Sun by more than 90º. Consequently, its ability to radiate heat away into deep space was improved.

Another method was a different charging scheme called "ratchet charging", known to have been successfully employed on the HST (Hubble Space Telescope). In this method the battery is charged more than once during a Sun phase and the EoC (End–of–Charge) value is raised every week. A software update during the latter stage provided more flexible control over the solar panel strings, allowing the satellite to fly without a pitch offset which was used earlier to lower the voltage.

Note: The NiH2 battery is the operational battery set of TET-1, while the Li-Polymer battery is one of the 11 technological experiments on TET-1.

• In the spring of 2014, the Firebird mission is prepared with most of the focus on the infrared camera. The bottleneck with regard to the execution of payload operations will be the ability to downlink the infrared pictures. Only as many pictures will be taken as the downlink capacity permits. Power is no constraint. In case the temperature becomes an issue again, additional actions could be taken by switching off units such as the star cameras or the magnetic coil system for certain periods.

- The star cameras had already been switched off for a portion of the orbit relatively early in the mission. This was effective temperature wise (~2ºC). However, since the IMU (Inertial Measurement Unit) was then the only source of accurate attitude information and that unit had failed several times throughout the mission it was decided to not continue this course of action. The software update had essentially fixed the IMU issue, though, and therefore the approach could be attempted again. The attitude requirements for the infrared camera will need to be taken into account, though.

- Building on the experience gained from operating GRACE would also be beneficial. At the moment, the battery treatment on that mission is continuously optimized. Although the rationale behind the treatment is understood the data necessary to do the same on TET is not available. If this changes in the future a similar optimization could possibly be performed. Implementing the Grace treatment much earlier in its life on TET could significantly extend its life expectancy.

• Starting on November 1, 2013, TET-1 is being used as the first satellite of the FireBird mission of DLR (German Aerospace Center). 37)

All payloads are being operated successfully. During the 3rd TET Customer Days in March 2013, the payload providers presented also their scientific and technology (interim) results. The occasion provided information on the on-going TET-1 mission through current results and feedback from the experimenters. Further improvements to mission operations have been discussed, and a programmatic outlook on future OOV (On-Orbit Verification) missions completed the program.

The FireBird mission consists of the two constructed DLR spacecraft TET-1 and BIROS. The main goal of this mission is the detection and monitoring of so called high temperature events (HTE), e.g. forest fires or other hot spots.

1) The TET-1 spacecraft (launched June 22, 2012) was in the first year of operations used by the OOV (On-Orbit Verification) mission of the DLR Space Administration. After one year in space, the objectives changed to the goals of the FireBird mission, supported by the R&D program of the DLR Directorate Space.

2) The BIROS (Bi-spectral Infrared Optical System) spacecraft, with a launch expected in 2015, is the second satellite of the two satellite constellation FireBird. BIROS is also supported by the R&D program of the DLR Directorate Space. The constellation phase of FireBird is expected to last one year. The project "FireBird" ends in December 2015, but if the satellites are still working fine it is perhaps not the end of the mission.

3) BIROS has the same size as TET-1 (but with a mass of 140 kg) and an IR payload for fire detection. The secondary payloads on BIROS are:

• AVANTI, an optical navigation experiment with BEESat-4, a CubeSat of the TUBerlin. The CubeSat seves as non-cooperative target for optical navigation and proximity operations (100 m to 10 km distance). The ISL (Intersatellite Link) BEESat-4-BIROS permits the usage of the CubeSat GPS receiver as a BIROS "remote device" for AVANTI verification.

• VAMOS (Verification of Autonomous Mission Planning On-board a Spacecraft). VAMOS, a GSOC experiment, will be used to schedule and (re-)command tasks. 39)

• OSIRIS, an experimental optical communication system for small satellites. 40)

• The TET-1 spacecraft and its payload are operating nominally in 2013. Since October 16, 2012 TET-1 is fully operational and is undergoing routine mission operations. 41)42)

• October 16, 2012: The TET-1 satellite started its operational phase after a review board gave its permission. 43)

• August 10, 2012: Continuation of the commissioning phase (testing of all subsystems). The LEOP phase was ended 1 week after launch. 44)

• July 24, 2012: In LEOP (Launch and Early Orbit Phase), DLR/GSOC conducted initial test with the spacecraft subsystems involving in particular the various attitude modes. 45)

Experiment complement in the Payload Segment:

The payload segment contains the payload itself, parts of the AOCS and one of the low gain antennas. Due to the design the sole mechanical and thermal interface between payload and satellite bus is the payload platform, which allows an easy and fast integration of the pre-assembled payload. - This payload platform can be designed as an optical bench (like on BIRD), but in the TET-1 mission it was not required. 46)

For TET-1, a total of 11 experiments have been selected which are covering a range of technologies. The experiments are developed by the individual providers according to specific TET standards and are thoroughly acceptance tested at Kayser-Threde before integration in the payload segment of the satellite.

• Three different types of next generation solar cells including thin-layer technology (Figure 29).

• Lithium polymer battery

• Two GPS receiver systems

• Sensor bus system

• Demonstration of a picosatellite propulsion system: Aquajet

• Test of a new infrared camera as follow-up of the BIRD camera system

The accommodation of the experiments in the Payload Segment is shown in Figure 24. The accommodation has been driven by the various experiments needs as e.g. viewing directions, unobstructed view angle for antennas, heat dissipation, etc. Thermal analyses, power budget and data handling considerations were performed to establish mission scenarios which are fulfilling the individual operational requirements of the experiments as e.g. number of experiment cycles, durations and timelines which are compliant with the satellite resources. The solar cell experiments have been accommodated on the sun pointing side of TET-1 next to the satellite bus solar arrays (Figure 29).

Figure 22 shows the block diagram of the CPU (Central Processing Unit) module and Figure 23 the top side of the CPU board. The central parts are the radiation tolerant LEON-III-CPU and a radiation tolerant system FPGA (RTAX-FPGA). These two components contain with exception of the memory devices and the physical interfaces all essential logic elements.

Figure 23: Top side view of the CPU module (image credit: Kayser-Threde)

The CPU LEON3-FT, CID-7 type with FPU of Gaisler Research has been used. This is a failure tolerant version of the LEON-3 CPU, which is integrated in an Actel RTAX-2000 FPGA This configuration has a performance of 20 MIPS. The LEON3-FT CPU is based of the SPARC-V8 architecture.

The system-FPGA contains the following functions:

- Memory interface for SDRAM with Reed-Solomon error correction and DMA controller for die image data of the SpaceWire interfaces.

- 2 asynchronous serial interfaces (UART)

- 2 synchronous serial interfaces for the telemetry-IF of the CPU modules

- 1 fast serial interface (SpaceWire) for control of the digital boards

- 2 interfaces for the PPS signals of the CPU modules

- 1 SpaceWire core for the communication with the digital-IO module

- Latch-up protection logic.

The SDRAM memory on the CPU module amounts to 768 MByte and is split into two banks. The useful payload data memory amounts to 512 MByte; the remaining 256 MByte are used for error correction. The CPU module contains two EEPROMS 28LV011 from Maxwell with a memory size of 1 Mbit each. In one EEPROM the Bootloader is located, in the other one the Flashloader. Two flash memory modules with a capacity of 2 GB are on the CPU module. The second one is for redundancy purpose.

PSS (N6) provides the interface between the satellite bus and the experiments, a fixed and a flexible part that can easily be adjusted to the experiments needs. The overall objective of the experiment are: 49)

• Reduction of mass

• Provision of more flexibility regarding design and integration

• Simplified assembly, integration and test procedures.

Figure 25: Schematic view of PSS (image credit: Kayser-Threde)

PSS, developed by Kayser Threde, is a modular system consisting of a backplane and set of different types of boards in Europe Card size (160 mm x 100 mm):

- Two processor boards (1 main, 1 cold redundant) used for all PSS activities, for control of the experiments and for communication with the satellite

- One digital and one analog I/O board providing the electrical interfaces to the experiments The range of interfaces comprises serial communication with RS-422, I2C and SpaceWire, general purpose digital inputs and outputs, outputs for the distribution of PPS-signals, high precision differential analog inputs and inputs for temperature sensors of type AD590 or PT1000.

- Five power boards for regulated for unregulated voltages for supply of the experiments and the needs of the PSS. All power channels can be switched on/off independently by relays, all channels are protected by latched current limiters and they are equipped with sensor circuits for monitoring voltages and currents. For TET-1 regulated (6) and unregulated (8) voltages with up to 8 A of current are provided.

Figure 26: Testing of the payload supply system in a test rack (image credit: Kayser-Threde)

The PSS is accommodated in the Electronic Segment of the satellite bus next to the satellite bus controller. The electronic segment provides space for 12 boards in 3 stacks as shown in Figure 26. Thus, it was possible to integrate one of the experiments, the N17 board within the Payload Supply System. The block diagram (Figure 27) shows the PSS with the various interfaces.

BOSS (Embedded Operating System design for dependability and for formal verification)

BOSS is a real-time operating system of BIRD microsatellite heritage (launch Oct. 22, 2001) which also provided the original name, namely BOSS (BIRD Operating System 'Simple') of the system. FhG/FIRST [Fraunhofer Gesellschaft/Institut für Rechnerarchitektur und Softwaretechnik (Institute of Computer Architecture and Software Technology), Berlin Adlershof, Germany] is the developer of the BOSS system. The SBC (Spacecraft Bus Computer) of TET-1 employs an an advanced version of the original BOSS system that assumes the transmission, processing, and memory of all data on board the satellite. In addition to data acquisition from individual subsystems, such as the experiments conducted on board or the power supply, it also takes over communications with ground control. With its multitude of interfaces to subsystems, components, and application software, the SBC constitutes a highly complex system with the following features: 50)

• Redundancy: A highly redundant architecture with a quadruple computer node is being used. Each computer node is providing redundant processing and memory structures. To prevent data loss in case of memory errors, each computer node is equipped with a shadow storage. The content of the shadow storage will replace the memory content should an error be detected in a node. The control and communication unit is located in a FPGA (Field-Programmable Gate Array).

• Latch-up protection: Radiation in particular presents a major challenge to computer systems on board satellites. High-energy particles are constantly impinging on the satellite computer's components which can lead to a SEU (Single Event Upset). In a SEU, a so-called bit flip, in which the state of a bit is altered, can cause a malfunction of the affected component. To avoid this, FhG/FIRST is employing radiation-tolerant components as well as a latch-up protection: through continuous voltage metering in various modules of the computer, a short-term shutoff of the affected component is carried out in case of disproportionately high voltage rises, thus preventing destruction.

• EDAC (Error Detection and Correction subsystem): The EDAC of the SBC works according to the monitor-worker principle: two active computer nodes work together and monitor each other. A malfunction is for instance detected by one computer node not sending data at all, or sending obviously false data, that do not match the other computer node's results. The computer node receiving such false data or none at all, detects this and prompts a reset of the respective other computer node. - Furthermore, each computer node can opt for a reset itself when it detects contradictory data. In the extreme case of one or both computer nodes delivering differing results even after a reset, the remaining computer nodes take over control. As long as they are not needed, they form the so-called emergency reserve: during this time, they are inactive and thus not subject to radiation-induced aging.

The microprocessor board has dimensions of 100 mm x 170 mm x 20 mm, a mass of 0.166 kg, a radiation hardness of > 13 kRad (tested), a power consumption of 4 W (max), and a temperature range of -40º to 80ºC.

The objective of NOX (N16) is to demonstrate and validate the use of a COTS-based dual-frequency GPS receiver, originally designed for terrestrial use only, can be successfully employed in Low-Earth Orbit (LEO) satellite applications. As a secondary objective, the technical characterization of the receiver is supplemented by the collection of scientific GPS measurements and their use in two key applications: precise orbit determination (POD) and occultation measurements. 51)52)

The NOX payload comprises a PolaRx2 GPS receiver developed by Septentrio, Belgium. The PolaRx2 receiver is built around the GNSS Receiver Core (GreCo), which represents an advanced version of the earlier AGGA0 chip. It offers a total of 48 channels and can thus track C/A code, P1 code and P2 code for up to 16 GPS satellites. The tracking and navigation software is executed on a MachZ system-on-a-chip computer that includes an i486 core and can be operated at up to 128 MHz.

For use within the NOX experiment, the PolaRx2 receiver is complemented by a dedicated interface board (Figure 30), which performs the power conditioning and latch-up protection and provides the necessary line drivers for serial communication and discrete signal. The total mass of NOX is 1 kg, power consumption of 8 W.

Aside from the electronic box, the NOX payload comprises a switchable pair of GPS antennas, one pointing to the zenith and used for POD, and a second one oriented in anti-flight direction (pointing at the Earth horizon) devoted to occultation measurements. The switching between these antennas is performed via an R/F relay. A dual-frequency low-noise amplifier is used to amplify the signals received by the passive antennas to a level suitable for processing by the GPS receiver.

Aquajet:

Aquajet is a small satellite propulsion system designed and developed at AI (Aerospace Innovation GmbH), Berlin in close cooperation with the Aerospace Institute (ILR) at TUB (Technical University Berlin). The objective is on-orbit qualification/verification of the Aquajet system performance on the TET-1 mission. The Aquajet micropropulsion device is an enabling system, small enough to provide its services to future pico- and nanosatellite missions. In particular, the micropropulsion device is an enabler for the positional control of nanosatellite constellations. 53)

The following parameters pertain to Aquajet:

• The resistojet propulsion system is based on environmentally benign ("green") and safe propellants (water & anti-freeze)

• The system is ECSS (European Cooperation for Space Standards) compliant in development and testing procedures

The optical payload, designed and developed at DLR, consists of an assembly of three pushbroom cameras, one in the VNIR (Visible Near Infrared) range and two imagers in the infrared region. The overall objective is the detection and quantitative analysis of HTE (High Temperature Events) like wildfires and volcanoes. 54)55)56)

Time is essential to support most effectively the decisions of fire managers in fire suppression planning, crew mobilization & movement. Therefore, on-board processing of fire front attributes, including geo-referencing and their direct transmission to the user on ground is a challenging task for small satellites, but it shall be technically feasible.

Key procedures for on-board fire detection and analysis are pre-processing and extraction of fire attributes. The pre-processing includes:

- Radiometric correction (using system correction files)

- Inter-channel co-registration (using system correction files), and

- Geo-referencing (using on-board navigational information).

The fire detection and analysis extraction of fire attributes includes:

The on orbit verification of on-board fire detection and analysis will be tested on Germany's first Technologie Erprobungs Traeger (TET-1). The optical payload of TET-1 is of BIRD mission heritage (launch of BIRD in 2001) together with a special signal processor to demonstrate the on-board processing of fire attributes based on the sensor data obtained over wildfires.

BIROS (Berlin InfraRed Optical System) is a follow-on fire detection mission of DLR with a planned launch in 2016. BIROS will be based on the satellite bus as developed for TET-1.

The BIROS sensor system is nearly identical to the optical payload of TET-1. It will be built – as the BIRD and TET-1 sensors which were designed by the DLR Institute of Robotics and Mechatronics / Dept. of Optical Information System (DLR-RM-OS) in Berlin-Adlershof.

KERAMIS (Keramische Mikrowellenschaltkreise für die Satellitenkommunikation) stands for the continued R&D activities of an industrial-academic consortium of partners experienced in the design, development, and fabrication of compact microwave modules based on the ceramic multilayer technology LTCC (Low-Temperature Cofired Ceramic) for satellite communications at Ka-band frequencies. The overall objective is the development of innovative and inexpensive components for future applications in multimedia satellite communications. This technology project (N18) is funded by DLR and BMWi (Bundesministeriums für Wirtschaft und Technologie) to be flight-validated on TET-1. 57)58)59)

The LTCC technology offers great potential for cost-efficient three-dimensional hybrid-integrated hermetic modules of high functional density incorporating active and passive sub-modules and circuitry. Specific challenges of the project are:

• to control the high structural precision required for industrial components for spaceborne microwave applications

• to balance fabrication issues and microwave performance

• to demonstrate full space qualification.

For these reasons, specific flight experiments have been developed and implemented on board of a test satellite for remote testing in space. A contribution to the competitiveness of the German-European space program as well as the approval of innovative modular microwave design can be considered essential outcomes of the satellite flight.

Three experiments have been developed as payload equipment for OOV (On-Orbit-Verification) in a national R&D LEO satellite (TET-1):

The University Hamburg-Harburg is responsible for conversion of a transmitted analog S-band signal to Ka-band and vice versa; this conversion concept test is the stated objective of the payload module. To this end, different transceiver configurations are designed, which consist of previously developed basic LTCC modules like mixers, signal sources and power amplifiers. The general suitability and versatility of advanced LTCC packaging in combination with hybrid integration methods for high reliability applications will be verified.

An Actel ProASIC AEPE3000L in FG896 ball grid housing contains the whole digital logic. This is mainly a LEON processor with its periphery. The high pin count allows the direct connection of the whole memory array with its 4 data and control busses independent for each memory partition. So, each partition can be switched on and off independently. The width of the data bus of 32 bits is chosen to fit to the bus width of the processor. The LU switches are controlled by the microprocessor.

Figure 42: Photo of the MORE PFM without cover (image credit: TU Braunschweig)

Qualification of Lithium-Polymer battery:

The Li-Po battery test is being implemented by ASP (Advanced Space Power Equipment GmbH) of Salem-Neufrach, Germany. The objective of the experiment is to space qualify the following items: 63)

A redundant battery management system is provided to monitor all aspects of battery operations.

TFSC (Thin Film Solar Cell) experiment:

The TFSC test is developed by Solarion AG, Leipzig, and HTS GmbH, Coswig, Germany (actually a larger partnership from an academic and industrial background). The objective is to use flexible CIGS (Copper Indium Gallium Selenide) cells on a plastic substrate for power generation and to test their performance in the space environment. Of all thin film technologies, those based on CIGSe have the highest potential to reach attractive photovoltaic conversion efficiencies and combine these with low weight in order to realize high power densities on solar cell and generator level. The use of a flexible substrate ensures a high packing density. The demonstration of this enabling technology in space is of great interest because it offers possible applications in such niches as for space power satellites, solar sails or exploration missions. 64)65)

The TET-1 spacecraft is being monitored and controlled at DLR/GSOC using the DLR-owned S-band ground stations in Weilheim, Neustrelitz and SKT (Saskatoon) of CSA (Canadian Space Agency), the latter for the LEOP phase only. A PDC (Payload Data Center) will be provided at the location of the data reception station in Neustrelitz. The main tasks are acquisition, extraction, processing (formatting to a generic ASCII format) archiving and provision of payload data. The 11 users with their experiments onboard TET-1 retrieve (download) their data via a secure FTP interface (Ref. 46). 66)67)

For the first year of operations the routine operations network is based on WHM and NST ground stations with 4 scheduled contacts per day, 1 contact over WHM and another 3 over NST. The WHM contact is the contact also used for uploading of telecommands while the 3 other contacts are used for payload data dump and telemetry only. If required, stations can be used interchangeable - increasing reliability of the ground segment.

Figure 45: Overview of the TET ground segment (image credit: DLR)

During the first year, all 11 payloads are operated by GSOC according to their respective requirements – which may be e.g. number of activation cycles, hours of operation or certain attitudes. Requirements have been collected by KTH (Kayser-Threde) from the payload owners. To generate a valid operations concept also the satellite bus capabilities must be collected and analyzed. All information together is stored in an Excel sheet, containing all payload activities for the first year of operation.

It is expected that the satellite will be significantly longer operational than the first planned year. Operations during an extended mission phases are not yet defined but the focus is placed on operations for the N15 infrared camera. Some other experiments may also be operated but only on request of the payload owner and if the payloads still operational.

Experiment types

Specific operations requirement

Experimental battery

High energy consumption during battery loading

Different types of solar cells

Sun pointing

Memory cells

Nothing specific

Picosatellite propulsion system

Specific attitude to avoid satellite bus contamination

Radio transmission experiment

No regular TM/TC transmission possible during experiment

Experimental GPS receivers

Operations with good GPS satellite view which requires an attitude different from sun pointing

Infrared camera

Nadir pointing, high power consumption and high data generation during operations, satellite rotation about nadir axis by 180º for star camera operations

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).